Method and cell for conversion of dinitrogen into ammonia

ABSTRACT

The invention relates to a method and an electrochemical cell comprising a cathodic working electrode comprising a nanostructured catalyst, a counter electrode and an electrolyte for the reduction of dinitrogen to ammonia. The invention includes introducing dinitrogen and a source of hydrogen to the electrolyte, wherein the dinitrogen is reduced to ammonia at the cathodic working electrode. The electrolyte comprises one or more liquid salts formed from the combination of a specified set of cations and a specified set of anions.

FIELD OF INVENTION

The present invention relates to an electrochemical apparatus and methodfor the conversion of dinitrogen (N₂) into ammonia.

In one form, the invention relates to the cathodic reduction ofdinitrogen.

In one particular aspect the present invention is suitable for use inindustrial production of ammonia.

BACKGROUND ART

It is to be appreciated that any discussion of documents, devices, actsor knowledge in this specification is included to explain the context ofthe present invention. Further, the discussion throughout thisspecification comes about due to the realisation of the inventor and/orthe identification of certain related art problems by the inventor.Moreover, any discussion of material such as documents, devices, acts orknowledge in this specification is included to explain the context ofthe invention in terms of the inventor's knowledge and experience and,accordingly, any such discussion should not be taken as an admissionthat any of the material forms part of the prior art base or the commongeneral knowledge in the relevant art in Australia, or elsewhere, on orbefore the priority date of the disclosure and claims herein.

Ammonia production is a highly energy intensive process, consuming 1-3%of the world electrical energy and about 5% of the world natural gasproduction. World production is currently around 200 million tonnesannually, reflecting the vast need for this chemical in agriculture,pharmaceutical production and many other industrial processes.

Ammonia is also being considered as a carbon-free solar energy storagematerial, due to its useful characteristics as a chemical energycarrier. Compared to other chemicals that could be used to store solarenergy (such as hydrogen), ammonia is safe, eco-friendly and, mostimportantly, produces no CO₂ emissions. Once stored in this form, theenergy is readily recovered via the ammonia fuel cell.

For more than a hundred years, ammonia has been produced from dinitrogenand hydrogen in the presence of an iron based catalyst at high pressuresand high temperatures according to the following reaction:

This process, known as the Haber-Bosch process has been of keyimportance in producing the inexpensive fertilisers that have supportedthe large global population growth over the past century. TheHaber-Bosch process uses very high temperatures and pressures, andrequires substantial amounts of energy in the form of natural gas, oilor coal for the production of the required hydrogen.

Given the need to feed a growing world population, whilst simultaneouslyreducing global carbon emissions, it is highly desirable to break thelink between industrial nitrogen-based fertiliser production and the useof fossil fuels. Therefore, there is intense interest in alternativepathways for ammonia synthesis.

The ideal system for the conversion of dinitrogen into ammonia would beeconomically feasible and easily scalable, would operate at ambientcondition and be coupled to renewable energy sources such as wind, hydroor solar. An electrochemical approach is potentially capable ofachieving this aim. The electrochemical reduction process involvesdinitrogen gas as a starting material and uses various aqueouselectrolytes or H₂ gas as the source of the W. The electrochemicalreduction of dinitrogen largely depends on the structure, components,and surface morphology of the electrocatalyst. (van der Ham et al,Chemical Society Reviews 43, 5183-5191 (2014)).

For example, Koleli & Ropke have investigated polyaniline electrodes inmethanol/LiClO₄/H⁺ solution and achieved a maximum current efficiency of16% at −0.12V (vs normal hydrogen electrode) at room temperature andelevated pressure. (Koleli, F. & Ropke, T. Applied CatalysisB-Environmental 62, 306-310, (2006))

By using a membrane electrode assembly based cell with Pt electrodes Lanet al have achieved an ammonia production rate of 1.14×10⁻⁵ mol·m⁻²·s⁻¹from air and water at ambient temperature and pressure and an overallcell voltage of 1.6 V. (Lan et al, Scientific Reports 3, (2013)).

Ammonia has also been produced using a mixture of N₂ and steam in amolten hydroxide suspension of nano-Fe₂O₃ at a cell voltage of 1.2 V andcolumbic efficiency of 35%. (Licht et al. Science 345, 637-640 (2014)).Although this advance shows great promise for competition with currentammonia industry processes, the high temperature used (˜200° C.) stillrequires significant input of heat and energy. So far, electrochemicalconversion has not been sufficiently successful to be considered as aviable replacement for the Haber-Bosch process. Furthermore,electrochemical conversion of the prior art has not reached sufficientlyhigh efficiency levels such as those exhibited by dinitrogen-fixatingbacteria. (Rosca et al, Chemical Reviews, 2009, 109, 2209-2244).

US patent application 2006/0049063 and U.S. Pat. No. 6,712,950 teach thesynthesis of ammonia gas by anodic reaction from nitrogen-containingspecies or dinitrogen gas, and hydrogen-containing species or hydrogengas in a non-aqueous liquid electrolyte such as a molten salt or anionic liquid. The method involves the production of the N₃ ⁻ ion in theelectrolyte and then the reaction of the N₃ ⁻ ion at the anode toproduce ammonia. This method is limited by the need for the medium to beselected such that it can dissolve useful amounts of the N₃ ⁻ to supportpractical rates of ammonia production.

U.S. patent application 2016/0138176 (Yoo et al) describes a method ofsynthesizing ammonia using an electrolysis cell containing an aqueous,or liquid electrolyte of an alkali metal (salt) or an ionic liquid. Thedisclosure includes teaching of the use of electrolytes formed from awide range of cations and anions (many of which do not form liquidsalts) and some of which are of limited stability and utility withregard to efficiency of ammonia synthesis.

There is therefore an ongoing need to identify new electrolytes andimproved methods for cathodic dinitrogen reduction leading to ammoniasynthesis.

In contrast to hydrogen generation or CO₂ reduction, very few prior artelectro-catalysts or photo-catalysts have been reported to exhibituseful activity for N₂ reduction. Little is known about the requirementsor possible mechanisms for such reactions. However it is known that tobecome commercially viable, electrochemical conversion of dinitrogen hasto overcome the obstacles presented by the high stability and chemicallyinert nature of dinitrogen.

Thus far, catalyst development has focused on catalysts for thermalsynthesis of ammonia. Current electrochemical catalysts for ammoniasynthesis have been limited to conductive polymers (Koleli, F. & Ropke,T. Applied Catalysis B-Environmental 62, 306-310, (2006)), meso Ni—Cualloy (Licht et al. Science 345, 637-640 (2014)), and commercial Pt/Ccatalysts (Lan et al, Scientific Reports 3, (2013)).

Metallic nano-catalysts have been widely used for fuel cells and CO₂reduction. It is well-known that the performance of nanocatalystsdepends on their particle size, morphology and crystal structure.

Greenlee et al report the synthesis of a NiFe nanocatalyst forelectrochemical ammonia synthesis (Nov. 8-13, 2015, AlChE annualmeeting, Salt Lake City, Utahhttps://aiche.confex.com/aiche/2015/webprogram/Paper422270.html).Further characterisation or performance of the catalyst was notreported.

Prior art electrocatalysts typically do not have high enough efficiency,catalytic activity and stability for dinitrogen reduction. In addition,the low solubility of dinitrogen in water (20 mg/L, 20° C., 1 bar) leadsto low reaction rates in prior art reports.

SUMMARY OF INVENTION

An object of the present invention is to provide an efficientelectrochemical process for production of ammonia.

Another object of the present invention is to provide an electrochemicalcell suitable for an electrochemical process for cathodic dinitrogenreduction.

Another object of the present invention is to provide an improved methodfor cathodic dinitrogen reduction.

A further object of the present invention is to alleviate at least onedisadvantage associated with dinitrogen cathodic reduction processes ofthe prior art.

It is an object of the embodiments described herein to overcome oralleviate at least one of the above noted drawbacks of related artsystems or to at least provide a useful alternative to related artsystems.

In a first aspect of embodiments described herein there is provided acell for electrochemical reduction of dinitrogen to ammonia, the cellcomprising:

-   -   a cathodic working electrode comprising a nanostructured        catalyst for reduction of dinitrogen,    -   a counter electrode, and    -   an electrolyte comprising one or more liquid salts in contact        with the working electrode wherein the liquid salt is formed by        a combination of:        (i) a cation selected from the group consisting of PR₁₋₄        (phosphonium), NR₁₋₄ (tetra alkylammonium), C₄H₈NR₂        (pyrrolidinium) wherein each R group is independently linear,        branched or cyclic and preferably comprises from 1 to 18 carbon        atoms, optionally partially or completely halogenated,        optionally including a heteroatom, optionally including a        functional group preferably chosen from ethers, alcohols,        carbonyls (acetates), thiols, sulphoxides, sulphonates, amines,        azos or nitriles, and wherein two R groups may connect to form a        monocyclic or heterocyclic ring; and        (ii) an anion selected from the group consisting of        (RO)_(x)PF_(6-x) (phosphate), (RO)_(x)BF_(4-x) (borate),        R′SO₂NSO₂R′ (imide), R′SO₂C(SO₂R′)(SO₂R′) (methide), FSO₂NSO₂F,        C₂O₄BF₂, C₂O₄PF₄, RC₂O₄BF₂, RC₂O₄PF₄, CF₃SO₃ (triflate), R′SO₃        (sulphonate), R′CO₂, (carboxylate), CF₃COO (trifluoroacetate),        R′_(x)PF_(6-x) (FAP), R′_(x)BF_(4-x) wherein each R′ group is        independently linear, branched or cyclic and preferably        comprises from 1 to 18 carbon atoms, optionally partially or        completely fluorinated and optionally including a functional        group, preferably chosen from ethers, alcohols, carbonyls        (acetates), thiols, sulphoxides, sulphonates, amines, azos or        nitriles and wherein two R′ groups may connect to form a        monocyclic or heterocyclic ring.

In a preferred embodiment the liquid salt is formed by a combination ofa cation selected from the group consisting of C₄mpyr (butyl-methylpyrrolidimium), P_(6,6,6,14), (trihexyl tetradecyl phosphonium),P(C₂R_(f))₄ (where R_(f) is a perfluoroalkyl); and an anion selectedfrom the group consisting of eFAP (C₂F₅PF₃), NfO (nonafluorobutanesulphonate), PFO (perfluorooctane sulphonate), FSI(bis(fluorosulphonyl)imide, NTf₂ (bis(trifluoromethylsulphonyl)imide),B(otfe)₄ (tetrakis(2,2,2-trifluoroethane)borate and CF₃COO(trifluoroacetate).

In a particularly preferred embodiment, the liquid salt includes acation selected from PR₁₋₄ (phosphonium) cations.

In a particularly preferred embodiment, the liquid salt includes ananion selected from RSO₃ (sulphonate) cations, particularlyperfluorobutanesulphonate or perfluoropropanesulphonate, ortrifluorophosphates, particularly eFAP(tris(perfluoroethyl)trifluorophosphate).

In another preferred embodiment the cation and/or anion of the liquidsalt is fluorinated or perfluorinated.

It is also particularly preferred that the liquid salt has a relativelyhigh nitrogen solubility (compared, for example, to liquid salts of theprior art such as imidazolium salts and nitrile based anions such asdicyanamide)

In a particularly preferred embodiment of the cell for electrochemicalreduction of dinitrogen to ammonia, the reduction of dinitrogen toammonia occurs principally in a region adjacent a three phase boundaryon the working surface of the cathodic working electrode.

Typically the nanostructured electrocatalyst is applied to the workingsurface of the cathodic working electrode to create thegas/electrolyte/metal three phase boundary region where electrolysisprincipally takes place.

The electrolysis cell may include other features well known to those inthe art for carrying out electrolytic reactions and controlling thecurrent between the electrodes. For example, the electrolysis cell maybe adapted to control the temperature or pressure of operation usingwell known means such as heaters, cooling units or pressurising means.In addition the cell may include an ultrasonic generator for generatingsound waves of energy greater than 20 kHz.

The electrolysis cell preferably includes gas flow layers having thefunction of allowing introduction to the cell of a stream of gascomprising nitrogen and hydrogen or water vapour, and exit of gascontaining ammonia. The ammonia is optionally collected external to thecell.

In a further embodiment an assembly may be formed when two or more cellsaccording to the present invention are stacked in series. One or more ofthe stacked cells may additionally be folded or rolled. Gas can beintroduced at any convenient location including from the “end” of thelongest dimension of the assembly or from either side of the assembly.

In a second aspect of embodiments described herein there is provided amethod for the electrochemical reduction of dinitrogen to ammonia, themethod comprising the steps of:

(1) contacting a cathodic working electrode comprising a nanostructuredcatalyst with an electrolyte comprising one or more liquid salts,wherein the liquid salt is formed by a combination of:(i) a cation selected from the group consisting of PR₁₋₄ (phosphonium),NR₁₋₄ (tetra alkylammonium), C₄H₈NR₂ (pyrrolidinium) wherein each Rgroup is independently linear, branched or cyclic and preferablycomprises from 1 to 18 carbon atoms, optionally partially or completelyhalogenated, optionally including a heteroatom, optionally including afunctional group preferably chosen from ethers, alcohols, carbonyls(acetates), thiols, sulphoxides, sulphonates, amines, azos or nitriles,and wherein two R groups may connect to form a monocyclic orheterocyclic ring; and(ii) an anion selected from the group consisting of (RO)_(x)PF_(6-x)(phosphate), (RO)_(x)BF_(4-x) (borate), R′SO₂NSO₂R′ (imide),R′SO₂C(SO₂R′)(SO₂R′) (methide), FSO₂NSO₂F, C₂O₄BF₂, C₂O₄PF₄, RC₂O₄BF₂,RC₂O₄PF₄, CF₃SO₃ (triflate), R′SO₃ (sulphonate), R′CO₂, (carboxylate),CF₃COO (trifluoroacetate), R′_(x)PF_(6-x) (FAP), R′_(x)BF_(4-x) whereineach R′ group is independently linear, branched or cyclic and preferablycomprises from 1 to 18 carbon atoms, optionally partially or completelyfluorinated and optionally including a functional group, preferablychosen from ethers, alcohols, carbonyls (acetates), thiols, sulphoxides,sulphonates, amines, azos or nitriles and wherein two R′ groups mayconnect to form a monocyclic or heterocyclic ring.(2) introducing dinitrogen and a source of hydrogen to the electrolyte,wherein the dinitrogen is reduced to ammonia at the cathodic workingelectrode.

The method may additionally include a step (3) comprising collectingammonia generated at the cathodic working electrode, separating theammonia from other liquids and gases present by using a separate trap orseparation unit.

Typically the dinitrogen is reduced at the cathodic working electrode toammonia in the presence of a source of hydrogen, preferably hydrogen gasor water. In a particularly preferred embodiment of the method, thedinitrogen gas is humidified with water vapour to a controlled degreeand then the humidified gas is passed in a stream over the cathode wherethe dinitrogen is electrochemically reduced to form ammonia.

Typically, when the source of hydrogen is water, the anodic counterelectrode converts the hydroxyl ions formed at the cathode into waterand oxygen.

The counter electrode may be placed in the same electrolyte as theworking electrode or alternatively, it may be separated by some meanssuch as an electrolyte membrane or separator material. In anotherembodiment the counter electrode may be located in a compartment whichoptionally contains a different electrolyte medium, such as an aqueoussolution.

The counter electrode reaction may be water oxidation or anotheradvantageous oxidation reaction such as sulphite oxidation.

Electrolyte

The electrolyte in contact with the working electrode may comprise oneor more salts, and those salts may be in the solid or liquid states, orcombinations thereof.

The electrolyte is typically in the form of a layer. Preferably theelectrolyte includes a spacer or electrolyte membrane (which itself mayact as an electrolyte), for example a polymer electrolyte such asNafion™ or a Nafion™-liquid salt blend, or a gelled liquid saltelectrolyte, or is an electrolyte soaked into a porous separator such aspaper or Celeguard™.

In a further embodiment of the present invention, there is provided anelectrolyte membrane comprising a thin layer of material combined withone or more liquid salts as herein described for use in the cell of thepresent invention.

Preferably the electrolyte of the present invention has high solubilityfor dinitrogen and low solubility for water. Preferably the dinitrogensolubility is at least 100 mg/L at the operating temperature andpressure of the cell and more preferably more than 200 mg/L; mostpreferably the solubility is greater than 400 mg/L. The solubility ofwater in the liquid salt is preferably less than 5 weight % at theoperating temperature and pressure of the cell, more preferably lessthan 2% and most preferably less than 1%.

Liquid Salt

Where used herein the term liquid salt is intended to refer to anelectrolyte medium that is liquid at the temperature of use and thatcontains one or more salts (each of which may be solid or liquid intheir pure states). The salts may be chosen from any suitable metalsalts, organic salts, protic salts, complex ion salts or the like.

The liquid salt medium can also be formed by mixing solid salts tocreate a liquid salt of the desired characteristics.

The liquid salt medium may contain additional components including wateror other molecular liquids that are miscible with the liquid saltelectrolyte. As will be appreciated by those skilled in the art, suchmaterials include dimethylsulfoxide, tetraglyme and other oligio- andpoly-ethers, glutaronitrile and other high boiling point nitriles,trifluorotoluene and other wholly or partially fluorinated solvents andpropylene carbonate and other carbonate solvents. Where the molecularliquid component is volatile, the Na gas stream can be pre-saturated bybubbling through a container of the molecular solvent prior to the gasstream passing into the reduction cell; any molecular liquid passing outof the cell in the product gas stream can be condensed or otherwisecaptured for re-use. Preferably the molecular liquid is fluorinated orperfluorinated. Preferably the molecular liquid is present in the liquidsalt medium at a level between 90 vol % and 0.1 vol %, more preferablybetween 20 vol % and 0.2 vol % and most preferably between 50 vol % and0.5 vol %.

The electrolyte comprising liquid salt provides an ion conductive, lowwater content medium in which the process reactions occur. Theelectrolyte of the present invention also offers the advantage of low,or zero volatility and some gases are more soluble in these electrolytescomprising liquid salts than in water. In particular, some electrolytescomprising liquid salts can provide an elevated solubility for N₂ gas(compared to aqueous and other electrolytes) thereby increasing theconcentration of N₂ at the electrolyte/electrode interface.

In another aspect of embodiments described herein, the present inventionprovides an electrolyte medium for electrochemical reduction ofdinitrogen to ammonia according to the method of the present invention,the medium comprising one or more liquid salts having high solubilityfor dinitrogen and low solubility for water.

In a preferred embodiment the electrolyte comprises one or more eFAPliquid salts because they exhibit high N₂ solubility (ref S. Stevanovicet al, Chem. Thermodynamics 59 (2013) 65-71).

The electrolyte comprising liquid salt can also exhibit a degree ofhydrophobicity that offers a controlled water activity to theelectrochemical reaction, that is high enough to support the N₂reduction, but not so high as to support rapid reduction of water to H₂.In a preferred embodiment the electrolyte comprises one or morehydrophobic liquids based on the P_(6,6,6,14) cation. In a particularlypreferred embodiment the electrolyte is substantially comprised of theliquid salt, [P_(6,6,6,14)][eFAP].

In a particularly preferred embodiment the electrolyte is preconditionedprior to use, such as, by contacting it with an aqueous hydroxidesolution. Without wishing to be bound by theory, the preconditioning mayintroduce a trace amount of OH⁻ into the liquid salt that provides adefined proton activity in the electrolyte of the present invention.

The temperature range applied to the electrolyte materials in a cellaccording to the present invention may be between −35 and 200° C.,preferably 0° C. to 150° C., and most preferably 15° C. to 130° C.

The pressure range of application is typically between 0.7 bar nitrogenpressure to 100 bar nitrogen pressure, preferably 1 bar to 30 bar andmost preferably 1 bar to 12 bar. The pressure may be continuous, that isconstant, or constantly increasing over time. Alternatively the pressuremay be discontinuous, that is pulsed over time—alternating betweenhigher and lower pressure values within the aforesaid ranges.

A pressure-temperature combination can be chosen to allow elevated Nasolubility and electrochemical kinetics and also to allow ammonia to begenerated at close to, but below, its condensation point (for example 10bar at 25° C. or 40 bar at 79° C.).

Typically a continuous current will pass between the cathodic workingelectrode and the counter electrode, however in some applications suchas a wind power photovoltaic panel power driven process an intermittentor pulsed current may be suitable.

Nanostructured Catalyst

In a third aspect of embodiments described herein there is provided acatalyst for electrochemical reduction of dinitrogen to ammonia, thecatalyst comprising nanostructured materials having a highelectrochemical working surface area, as indicated by a double layercapacity, measured in an adjacent electrolyte layer of greater than 0.1mF/cm² and preferably greater 1 mF/cm².

Preferably the nanostructured catalyst comprises one or more metals inthe form of elemental metal or inorganic compounds comprising metals.The nanostructured catalyst may be in the form of discrete particles orsheet or film or three dimensional structure. The nanostructuredcatalyst embodies morphological features that may be of any shape withat least one dimension in the range of 1 nm to 1000 nm.

Suitable metals include any of the transition metals or lanthanidemetals including Fe, Ru, Mo, Cu, Pd, Ti, Ce and La as well as theiralloys with other metals and semimetals.

The aforementioned metals may be surface decorated with an oxide or asulphide of the metal, or a composite may be formed of the metal withits oxides or sulphides.

The catalysts may also comprise a metal complex consisting of two metalsbridged by sulphides. Preferably the metals are Fe and Mo.

The catalyst nanoparticle film may preferably be prepared by a cyclicvoltammetry or a pulsed voltammetry electrodeposition method.

In another preferred embodiment the catalyst may comprise conductivepolymer materials such as PEDOT.

In yet another preferred embodiment the catalyst may comprise dopedcarbon materials, particularly carbons doped with N and/or S.

The catalyst is preferably supported or decorated on an electricallyconductive, chemically inert support. Suitable supports includefluorine-doped tin oxide, graphene, reduced graphene oxide, porouscarbons, carbon cloth, carbon nanotubes, conducting polymers and porousmetals.

Faradaic efficiency is a particular deficiency of related processes ofthe prior art. Faradaic efficiency can be used to describe the fractionof electric current that is utilised in the N₂ reduction reaction. Theremaining fraction that is, (100−Faradaic efficiency)%, is consumed inundesirable side reactions including the production of H₂ and hydrazine.These bi-products represent wasted energy and may also require complexseparation methods from the desired product. It is one of the purposesof the present invention to provide a method of relatively high Faradaicefficiency preparation of ammonia.

Other aspects and preferred forms are disclosed in the specificationand/or defined in the appended claims, forming a part of the descriptionof the invention.

In essence, embodiments of the present invention stem from therealization that the efficiency of ammonia production can be improved bychoice of specific non-aqueous electrolytes that increase theconcentration of dissolved N₂ gas and play a certain homogeneouscatalysis role to increase the activity of dinitrogen in the reductionreaction while lowering the rate of undesirable competing reactions suchas H₂ production. The efficiency is further improved by using specificnanostructured catalysts.

Advantages provided by the present invention compared with the processesof the prior art comprise the following:

-   -   conversion of dinitrogen to ammonia with high Faradaic        efficiency;    -   the reaction can be carried out at ambient temperature and        pressure;    -   greater solubility of dinitrogen in the electrolyte;    -   increased activity of dinitrogen in the reduction reaction;    -   lower rate of undesirable competing reaction such as H₂        production;    -   water can be been used as hydrogen source so that no extra        hydrogen is needed;    -   suitable catalysts can be utilised from cheap and earth abundant        materials;    -   no highly corrosive electrolytes.

Further scope of applicability of embodiments of the present inventionwill become apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the disclosure hereinwill become apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

Further disclosure, objects, advantages and aspects of preferred andother embodiments of the present application may be better understood bythose skilled in the relevant art by reference to the followingdescription of embodiments taken in conjunction with the accompanyingdrawings, which are given by way of illustration only, and thus are notlimitative of the disclosure herein, and in which:

FIG. 1 is a schematic diagram showing a typical electrochemical cell forN₂ reduction according to the present invention;

FIG. 2 is a schematic depiction of an N₂ reduction cell according to thepresent invention and based on hybrid electrodes.

FIG. 3 is a schematic depiction of a stacked N₂ reduction cellarrangement according to the present invention showing cells asdescribed in FIG. 2 stacked in series connection.

ABBREVIATIONS

Where used herein the abbreviations refer to the following chemicalspecies:

B(otfe)₄—tetrakis(2,2,2-trifluoroethanoxy)borate (B(OCH₂CF₃)₄)C₄mpyr—butyl-methyl pyrrolidiniumEMIM—ethyl methyl imidazoliumeFAP—tris(perfluoroethyl) trifluorophosphateFSI—bis(fluorosulphonyl)imide ((FSO₂)₂N)HMIM—hexyl methyl imidazoliumNfO—nonafluoro butane sulphonate (CF₃(CF₂)₃SO₃)NTf2—bis(trifluoromethyl sulphonyl)amideP_(6,6,6,14)—trihexyl tetradecyl phosphoniumP_(1,4,4,4)—tri butylmethyl phosphoniumPEDOT—poly(3,4-ethylene dioxythiophene)perfluorobutyl sulphonate (F₉C₄SO₃)PFO—perfluoro octane sulphonate (F₁₇C₈SO₃)PFB—perfluorobutyrateR_(f)—perfluoro alkyl —(CF₂)_(n-1)(CF₃) where n=is an even number

DETAILED DESCRIPTION

The present invention will be further described with reference to thefollowing non-limiting examples. These examples explore theelectrocatalytic activities of the new catalytic materials in liquidsalt electrolytes.

The examples also illustrate the fabrication of electrodes based onvarious metal nanostructures (nanoparticles or films) using chemical andelectrodeposition methods and investigation of their electrocatalyticproperties in electrolytes comprising one or more liquid salts. By usingelectrolytes comprising liquid salts, dinitrogen was successfullyconverted to ammonia with Faradaic efficiency of >60% at ambienttemperature and pressure, which exceeds room temperature efficiencies ofprocesses of the prior art.

The method of the present invention utilising a liquid salt basedprocess not only can significantly increase the solubility of dinitrogenbut also play a certain homogeneous catalysis role to increase theactivity of dinitrogen in the reduction reaction and at the same timelower the rate of undesirable competing reactions such as H₂ production.

N₂ electroreduction catalyst films based on Fe were prepared byelectrodeposition of the Fe onto a commercially available fluorine-dopedtin oxide or porous metal (or carbon) substrates. Fluorine-doped tinoxide coated glass is electrically conductive and ideal for use in awide range of devices. Fluorine doped tin oxide is relatively stableunder atmospheric conditions, chemically inert, mechanically hard,high-temperature resistant, has a high tolerance to physical abrasion.Stainless steel mesh or cloth is useful as a high surface areasubstrate.

The deposition electrolyte comprised 10 mM FeSO₄, 10 mM NaOH and 10 mMcitric acid. The electrodeposition was controlled by a potentiostatusing a three-electrode system using fluorine doped tin oxide glass, Timesh, and a saturated calomel electrode (SCE) as the working, counterand reference electrode, respectively. A typical electrodeposition ofthis type is conducted using a cyclic voltammetry method between −1.8 Vand −0.8 V. A black and shining film was formed after several cycles.Changing the number of cycles allows control of the thickness of film,and a typical film was electrodeposited by 10 cycles at a scan rate of20 mV s⁻¹.

The film was characterised by scanning electron microscopy (SEM),transmission electron microscopy (TEM), energy-dispersive X-rayspectroscopy (EDX), and electrochemical surface area (ECSA) measurement.

N₂ Cathodic Reduction

The electrochemical N₂ reduction was conducted in a three-electrodesystem by using the Fe/fluorine doped tin oxide (or porous metla)electrode and platinum wire as the reference and counter electrodes,respectively. Cyclic voltammetry was measured in a one-compartment cell,while the N₂ reduction was conducted in a separated cell by isolatingthe counter electrode in compartment with a glass frit. Typicalreduction was carried out for 3 hrs at −1.2 V vs Pt.

The N₂ gas was bubbled through a water trap to saturate it beforebubbling into the electrolyte. The synthesized ammonia carried in theexit gas was trapped by passing the gas stream through a weak acidsolution (1 mM H₂SO₄)

Product Analysis

The ammonia produced in the gas phase was bubbled and collected through1 mM H₂SO₄ solution. The ammonia remaining in the ionic liquid aftercompletion of the electroreduction was extracted by washing the ionicliquid using 1 mM KOH. The total amount of ammonia is the sum of theammonia in the acid trap and the basic washing solution and was laterquantified using the indophenol blue method.

In a typical analysis, 0.5 ml of 0.5 M sodium hypochlorite was addedinto 0.5 ml aliquot of solution, then it was added 0.05 ml of 1 M NaOHwith 5% (by weight) of salicylic acid and 5% (by weight) of sodiumcitrate, and 0.01 ml of 0.5% (by weight) sodium nitroferricyanide. After2 hrs, the absorption of the solutions at 700 nm was measured on aUV-Vis spectrometer. Control experiments were conducted to validate theanalysis method by adding different concentrations of added NH₄Cl to the1 mM H₂SO₄ or 1 mM KOH solutions.

The Faradic efficiency was calculated based on 6-electron process, thatis;

N₂+6e ⁻+6H₂O⇔2NH₃+6OH⁻

FIG. 1 is a schematic diagram showing a typical electrochemical cell forN₂ reduction according to the present invention comprising a powersource (1), cathode (2), membrane (3) and anode (4). The counterelectrode reaction in the process may be water or hydroxide oxidation asillustrated. Alternatively where the desired product is the fertiliserammonium sulphate, the counter electrode reaction may be SO₃ ⁻² to SO₄⁻². An advantage of the latter is that the total energy cost of theprocess is lower than when oxygen is the cathode reaction product.

Cell

An N₂ reduction cell according to the present invention and based onhybrid electrodes is described in the following paragraphs. Thiselectrolysis cell is designed to optimally support electrolysis when thereactants which are one or more gases are only of low solubility in theelectrolyte and therefore it is desirable to have the reaction occurprincipally in the region at or near the three phase boundary (that is,the boundary between the electrode material, the gas phase and theelectrolyte) within the body of the hybrid electrode.

The hybrid electrode of this invention preferably comprises a porousconductive electrode material, optionally decorated with nanostructuredcatalyst, such that the electrode has a high electrochemically activesurface area and has a multiplicity of pores allowing both gas accessand electrolyte access within the body of the electrode.

In the present invention the gas remains adjacent one surface and thepores of the hybrid electrode are at least partially filled with the ionconducting material so that transfer of reactants and products occurs inthe region where the reduction of N₂ to NH₃ occurs.

This avoids one of the severe limitations of prior art electrodes thatare designed to support gas diffusion within the porous electrode; thisdiffusion process creates undesirable overpotentials and in-efficiencyin the cell in operation.

In this embodiment the cathodic working electrodes are hybridion-conducting electrodes or “electrode membranes” prepared by creatingporous metal membranes and impregnating the porous structure with an ionconducting material for example Nafion™ or a Nafion™ gel or a gelledliquid salt. Decorating the cathode with electrocatalysts creates agas/electrolyte/metal three phase boundary region where electrolysis cantake place.

The reactant gas is introduced directly to the electrocatalyst from theflowing gas. The reactant or product ions diffuse through theelectrolyte part of the electrode membrane and the gaseous products passdirectly into the flowing gas phase.

As shown in FIG. 2 the cell according to this embodiment is comprised oftwo porous electrodes (11,12) separated by an electrolyte layer (13).Electrode 1 is the cathodic working electrode at which nitrogen isreduced and electrode (12) is the counter electrode (anode) at whichwater oxidation or other desirable oxidation takes place. On the rearside of each electrode is a spacer layer (14, 15) which conducts gasesin and out of the cell.

The electrodes comprise layers of flexible and porous conductiveelectrode sheet materials that have the electro-catalyst (16) loadedonto the sheet and exposed directly to the gas layer (14,15)respectively. Optionally the catalysts (16) can be deposited throughoutthe structure of the electrode (11) to form a multiplicity of regionswhere gas, electrolyte, catalyst and conductive electrode come intocontact. The porosity of the conductor sheet is preferably such that thesheet is porous from one face to the other.

The porous conductive electrode material may be a macro- or micro- ornano-porous metal such as stainless steel, iron, nickel, copper,ruthenium or their alloys in the form of a mesh, foam or wool. Porouscarbon materials including carbon cloth, carbon particle/bindercomposites, graphene and reduced graphene oxide and conducting polymersare also suitable as porous conductive electrode materials.

The external encasement encloses the cell. In one form of the cell thecell is rolled or z-folded.

In a further embodiment of the present invention an assembly is formedby a number of cells stacked in series. In this format the electrolytein the cell layers (18) must be isolated from one another and theencasement (17) is a conductive material so that a series connection iscreated between cells. The stacked cells may also be folded or rolled.

The electrolyte can comprise a spacer or be in the form of anelectrolyte membrane, for example a polymer electrolyte, includingNafion™ or gelled Nation™ or gelled liquid salt electrolyte, or anelectrolyte soaked into a porous separator such as paper or Celeguard™type materials that are well known in the battery field.

In one form of the assembly, the layers (11, 13 and 12) are pressedtogether and then the electrolyte is gelled in-situ to produce a selfadhering assembly.

Gas flow layers (14) have the function of allowing the introduction of astream of flowing gas that contains nitrogen and water vapour. Theexiting gas contains ammonia which is optionally trapped external to thecell.

The gas flow layer (15) is designed to allow entry or exit of thegaseous reactions or products produced on that side of the cell,optionally a flow of air or other gas can be introduced to dilute andcarry this gas out of the cell. The gas flow layers (14,15) are createdeither by use of a highly porous separator or mesh, or by printing orotherwise creating a pattern on one or both sides of layer (17).

In the assembly of stacked cells the gas can be introduced at anyconvenient location including from the “end” of the longest dimension ofthe assembly or from either side of the assembly. Each of thesearrangements requires alternate sealing arrangements of layers (17).

The whole assembly may be as little as 1 mm thick, maximizing thesurface area and minimising the amount of electrolyte material needed.

Example 1. Preparation of Nanostructured Fe Catalyst

The following Example describes preparation of a nanostructured catalystsuitable for use in the present invention. A fluorine doped tin oxideelectrode (5×5 mm) was placed in an electrochemical cell and connectedas the cathode. A standard calomel electrode (SCE) acted as referenceelectrode and a Ti mesh served as the counter electrode. The electrolytesolution for production of the catalyst comprised 10 mM FeSO₄, 10 mMNaOH, and 10 mM citric acid in water.

Electrodeposition of Fe was carried out by cycling the potential of thefluorine doped tin oxide electrode between −0.8 and −1.8 V vs SCE at 20mV/s for 10 cycles. The electrodeposited Fe nanostructured layerappeared as a porous black layer comprising nanoparticles on thefluorine doped tin oxide. The deposited material is composed of mainlymetallic Fe with homogenous distribution of partially oxidized iron(with 10% oxides). The iron oxide forms during the anodic part of thedeposition cycle. The formation of iron oxide can control the ironnanoparticle size. The iron oxide is at least partially reduced duringthe initial stages of use as a cathode in ammonia production. TheFe/FeOxide composite may enhance the catalytic activity through asynergistic effect of the two oxidation states of iron.

The electrochemical surface area was determined by measuring the doublelayer charging current at 0 V vs Pt by cyclic voltammetry at 5 mV/s inbutyl methylpyrrolidinium eFAP liquid salt at room temperature. A doublelayer capacity of 2 mF/cm² was measured under these conditions.

Example 2. Reduction of N₂ in [P_(6,6,6,14)][eFAP]

The fluorine doped tin oxide electrode bearing the Fe electrocatalystlayer was placed in an electrochemical cell as the cathode. Theelectrolyte comprised the liquid salt [P_(6,6,6,14)][eFAP]. Theelectrolyte was preconditioned prior to use by shaking with 1 mM aqueousKOH solution. The aqueous solution was only sparingly soluble in theliquid salt, such that the excess solution can be removed as a separatelayer in a separating funnel.

The preconditioning may introduce a trace amount of OH⁻ into the saltsthat provide a defined proton activity in the electrolyte.

The counter electrode (Pt) was placed in a fritted compartment, whichalso contains the saturated liquid salt. The reference electrode is Pt.Dinitrogen gas at 10 ml/min was introduced into the cell using a bubblerdirecting N₂ bubbles over the cathode. The dinitrogen was pre-saturatedwith water by bubbling through a solution of low water vapour pressure.

The exiting gas flowing from the cell and containing unreacteddinitrogen and reduction products was directed into a further trapcontaining 1 mM H₂SO₄ to trap the ammonia produced as NH₄ ⁺. Thissolution was analysed for ammonia at the end of the electrolysis.

Electrolysis was carried out for 2 to 5 hours at voltages between −1.0to −2.0 V. At the end of the electrolysis the electrolyte was thricewashed with 1 mM KOH solution to extract any retained ammonia. The threewashes were also analysed for ammonia. The result was combined with theresult form the trap solution analysis. The Faradaic efficiency was 73%(+/−5%).

The solutions were also analysed for hydrazine but no hydrazine wasdetected (at LOD=5 μmol/L).

Example 3: Reduction of N₂ in [C₄Mpyr][eFAP]

N₂ reduction was carried out as described in Example 2 except that theelectrolyte comprised the liquid salt [C₄mpyr][eFAP]. The Faradaicefficiency in this case was 47% (+/−5%).

Example 4: Reduction of Na in [P_(6,6,6,14)][P₉C₄SO₃]

Following the same methods as Example 2, the electrolyte comprised theliquid salt [P_(6,6,6,14)][P₉C₄SO₃]. A constant current of 4 uA cm⁻² wasapplied for 2 hrs Ammonia measurement showed that the Faradaicefficiency for ammonia was 51%.

Example 5: Reduction of Na in [P_(6,6,6,14)][PFO]

Following the same methods as Example 2, the electrolyte comprised theliquid salt [P_(6,6,6,14)][PFO]. A constant current of 4 uA cm⁻² wasapplied for 2 hrs. Ammonia measurement shows that the Faradic efficiency% for ammonia was 39%.

Example 6: Reduction of Na Reduction with Assistance of Sonication

The following Example illustrates increased ammonia production byincorporating additional physical and/or-chemical processes.

Following the same procedure as Example 2, the whole electrochemicalcell was immersed into the bath of a sonicator. A constant potential of−1.2V vs NHE was applied on the working electrode for 30 min, andsonication was applied to the electrochemical cell to promote the Nareduction. The current increased by approximately 125% compared to itsprevious value in the presence of the sonication.

Example 7: Reduction of N₂ in the Presence of a Molecular LiquidComponent

As mentioned previously the electrolyte can by a combination of two ormore salts to create a liquid salt of the desired characteristics.Furthermore, the liquid salt may contain additional components includingwater or other molecular liquids as illustrated by the followingExamples.

Following the same methods as Example 3, trifluorotoluene 10% v/v wasadded into the electrolyte as a further component to decrease theviscosity of the liquid salt and promote the mass transport in theelectrochemical reaction. A constant potential of −1.2V vs NHE wasapplied for 30 min. The current increases by 50% compared to its valuein the absence of the trifluorotoluene.

Example 8: [C₄mpyr][perfluorobutanesulfonate]

[C₄mpyr][perfluorobutanesulfonate] has an elevated melting point (112°C.) and could be used for Na reduction at temperatures above this point.The melting point can also be lowered by the addition of a molecularliquid component as described in Example 7.

Example 9: [C₄mpyr][PFO]

[C₄mpyr][PFO] has an elevated melting point (87° C.) and could be usedfor Na reduction at temperatures above this point. The melting point canalso be lowered by the addition of a molecular liquid component asdescribed in Example 7.

Example 10: Preparation and Use of an Electrolyte Comprising a GelMembrane Containing a Liquid Salt

As described previously, the electrolyte used in the present inventionmay comprise a spacer or electrolyte membrane which itself is anelectrolyte, for example a polymer electrolyte, such as Nafion™ orgelled liquid salt electrolyte, or is an electrolyte soaked into aporous separator such as paper or Celeguard™. The procedure set outbelow in Example 10, while illustrative of preparation of a PVDF-HFPcopolymer gel electrolyte membrane, can be used more generally forpreparation of any gel membrane containing a liquid salt.

A PVDF-HFP copolymer gel electrolyte membrane was prepared as followsusing a liquid salt such as described in Example 3. A 9 to 1 ratio byweight of the liquid salt to copolymer was dissolved indimethylformamide and heated slightly. The solution was spread onto a 34mm diameter circle of Solupour™ membrane and left to dry. After drying,the excess gel on the outside of the membrane was removed.

A gas flow cell was constructed with a carbon paper/platinised carbonanode, the membrane and an Fe electrodeposited on stainless steel mesh(400 mesh) cathode. Hydrogen gas was introduced at the anode side of thegas flow cell and nitrogen gas was passed through the cathode side at arate of 180 mL/min each. The pressure of gas inside the cell was 1atmosphere. A constant potential of −1.2V relative to the anode wasapplied to the cathode for 30 mins.

The gases flowing through the cell were combined and bubbled through a40 mL solution of 1 mM H₂SO₄ and then through a 20 mL solution of 1 mMH₂SO₄ The gas was bubbled throughout the period of applied potential andfor 30 minutes after it was finished.

Ammonia produced over 30 mins of applied potential was 87.0 nmoles witha Faradaic efficiency of 24.9%.

Example 11: Preparation and Use of a Gel Membrane with [C₄Mpyr][eFAP]Liquid Salt Suitable for Use as a Free Standing Membrane

A membrane was prepared as for Example 10 except that the liquid salt topolymer ratio was 7:3 by weight and the solution was cast to form afree-standing membrane Ammonia produced over 30 mins of appliedpotential was 59.5 nmoles with a Faradaic efficiency of 71.5%.

Example 12: Preparation and Use of a Nafion™ Gel Membrane with a[C₄Mpyr][eFAP] Liquid Salt

A cell was constructed as described in Example 10, except that theelectrolyte comprised a Nafion™-gel membrane and was prepared asdescribed in the following example.

A solution containing 1 to 9 ratio by weight of Nafion™ and [C₄mpyr][eFAP] was prepared by mixing 9 parts of [C₄mpyr] [eFAP] and 20 parts ofa 5 w/w % solution of Nafion in lower aliphatic alcohols and water, assupplied by Aldrich. This was dropped on to a 34 mm diameter Solupour™membrane to achieve complete coverage of the solution over the membrane.It was then dried overnight in an oven at 60° C. The gas flow cell wasconstructed with this Nafion™-gel membrane between the cathode and theanode. Ammonia produced was found to be 93.1 nmoles with a Faradaicefficiency of 30.6%.

Example 13: Preparation and Use of a Free-Standing Electrolyte MembraneComprising a Nafion™ Membrane with a Liquid Salt

A cell was constructed as described in Example 11 except that themembrane was a Nafion™-gel membrane. The membrane was prepared asfollows. A 3 to 7 weight ratio mixture of Nafion™ and liquid salt wasprepared as described in example 12 and drop cast. It was then driedovernight in an oven at 60° C.

It will be apparent to the person skilled in that further variations ofthe cell described above can be constructed, such as, by excludingSolupour™ as the support for the electrolyte membrane or using othersupports.

Example 14: Na Reduction in [P6,6,6,14][PFB]

Following the same methods as Example 3, [P_(6,6,6,14)][PFB] was used asthe liquid salt electrolyte. A constant potential of 0.8 V vs NHE wasapplied for 3 hrs. The Faradaic efficiency for ammonia production inthis system was 18%.

Example 15: Na Reduction on Ru Catalyst

Following the same procedure as Example 3, a ruthenium film wasdeposited onto the working electrode. A constant potential of −0.8 V vsNHE was applied on the working electrode for 30 min. The current densitywas approximately twice that of Example 3 Ammonia measurement shows thatthe Faradaic efficiency for ammonia production in this system was 28%.

Use of Other Electrode Substrates

As the N₂ reduction currents obtained on the flat fluorine doped tinoxide glass substrate are too small to be practical, a variety of highsurface area and porous substrates such as nickel foam, stainless steelmesh were used to increase the working electrochemical surface area. Thecurrent density increases on using the porous substrates, for examplethe current increases from 0.012 A·m⁻² for fluorine doped tin oxide to0.1 A·m⁻² for stainless steel mesh, 0.25 A·m⁻² for nickel foam, withcorresponding Faradaic efficiency of 59% and 45% respectively. Due tothe increased current, the production rate increases remarkably from 2.9mg·m⁻² h⁻¹ on fluorine doped tin oxide substrate to 14 mg·m⁻² h⁻¹ on thestainless steel substrate. It is important to note that the stainlesssteel cathodes used here are quite thin and flexible so that there isconsiderable scope to further increase yields by optimizing cathodethickness and also to implement, high surface area, thin-layer or spiralwound type cell constructs.

Compared with [P_(6,6,6,14)][eFAP], the [C₄mpyr][eFAP] liquid saltproduces a higher current. This is probably due to its lower viscosity,which promotes better mass diffusion during the reaction. The Faradaicefficiency for N₂ reduction for the stainless steel electrode in[C₄mpyr][eFAP] is a little lower, as expected as the N₂ solubility inthis ionic liquid at 0.20 mg/g is lower than [P_(6,6,6,14)][eFAP] at0.28 mg/g. Nonetheless, the yield is higher in [C₄mpyr][eFAP].

The reason for the high solubility of N₂ in the selected ionic liquidsis further examined by DFT calculations. The interaction of variouscommon anions with N₂ is relatively weak with ions such as Cl⁻ and withfluorinated anions such as BF₄ ⁻ and PF₆ ⁻. On the other hand, theinteraction with [eFAP] is distinctly stronger.

Without wishing to be bound by theory it may be that two modes aredistinguished in the calculations; the first reveals the N₂ interactingwith the F-atoms attached to the alkyl chains, while the second has theN₂ interacting with the F atoms bound to the phosphorous, the latterbeing the stronger interaction. It appears from all of thesecalculations that the more strongly delocalized the charge is onto theneighbouring groups, the stronger is the N₂ binding interaction. Thedistinction between the binding energy in this eFAP complex and thatwith PF₆ is striking; in the eFAP case the additional delocalization ofcharge onto the three C₂F₅ groups modulates the charge on the P-boundfluorines and thereby enhances the interaction with N₂. Introducing thecation into these calculations shows that the interaction becomes evenstronger because of further charge interaction between the anion and thecation.

Comparative Examples

While the prior art includes general disclosure of the use ofelectrolytes comprising liquid salts, many of these electrolytes areunsuitable and/or have an untenably low Faradaic efficiency forreduction of dinitrogen according to the method of the presentinvention.

This can be illustrated, for example with reference to U.S. Patent2016/0138176 (Yoo et al) which describes a method of synthesizingammonia using an electrolysis cell containing an aqueous, or liquidelectrolyte of certain alkali metal (salts) or an ionic liquid. Inparticular, Comparative Examples 16 and 17 relate to salts disclosed inUS 2016/0138176 however in the method of the present invention, the saltof Comparative Example 16 results in an untenably low Faradaicefficiency while the salt of Comparative Example 17 becomes unstable.

Comparative Example 18 illustrates more generally that certain ionicsalts become unstable when used in the method of the present inventionand for that reason are unsuitable.

Comparative Example 16: Reduction of N₂ in [HMIM] [NTf₂]

N₂ reduction was carried out as described in Example 2 except that theliquid salt comprised the electrolyte [HMIM] [NTf₂] and the workingelectrode employed the catalyst of Example 1. Since ionic liquids aregenerally regarded as salts having a melting point <100° C. [HMIM][NTf₂] may be regarded as an ionic liquid. As the melting point of thissalt is around 70° C. the reduction was carried out at 70° C. in theliquid state.

The Faradaic efficiency in this case was only 0.64%.

Comparative Example 17: Reduction of N₂ in [EMIM] [B(CN)₄]

N₂ reduction was carried out as described in Example 2 except that theelectrolyte comprised the liquid salt [EMIM][B(CN)₄] and the workingelectrode employed the catalyst of Example 1.

The apparent Faradaic efficiency in this case was only 14%. However thesame apparent Faradaic efficiency was measured when an argon gas feed tothe cell was used, replacing the N₂ feed. Since in the latter case theammonia is not produced from the supplied N₂ it is being formed bydecomposition of the electrolyte comprising the liquid salt, indicatingthat such cyano-group containing liquid salts when used as electrolytesare not stable against reduction under these conditions.

Comparative Example 18: Reduction of N₂ in [EMIM][eFAP]

N₂ reduction was carried out as described in Example 2 except that theelectrolyte comprised the liquid salt [EMIM][eFAP] and the workingelectrode employed the catalyst of Example 1.

Ammonia production measurements show that both argon and N₂ saturatedelectrolytes produce ammonia (or a detectable amine), and the Faradaicefficiency % is 35% and 16% respectively. This indicates that the ionicliquid decomposes during the electrolysis to produce amine products andammonia. This liquid salt is therefore not suitable for nitrogenreduction to ammonia.

While this invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification(s). This application is intended to cover any variationsuses or adaptations of the invention following in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

As the present invention may be embodied in several forms withoutdeparting from the spirit of the essential characteristics of theinvention, it should be understood that the above described embodimentsare not to limit the present invention unless otherwise specified, butrather should be construed broadly within the spirit and scope of theinvention as defined in the appended claims. The described embodimentsare to be considered in all respects as illustrative only and notrestrictive.

Various modifications and equivalent arrangements are intended to beincluded within the spirit and scope of the invention and appendedclaims. Therefore, the specific embodiments are to be understood to beillustrative of the many ways in which the principles of the presentinvention may be practiced. In the following claims, means-plus-functionclauses are intended to cover structures as performing the definedfunction and not only structural equivalents, but also equivalentstructures.

“Comprises/comprising” and “includes/including” when used in thisspecification is taken to specify the presence of stated features,integers, steps or components but does not preclude the presence oraddition of one or more other features, integers, steps, components orgroups thereof. Thus, unless the context clearly requires otherwise,throughout the description and the claims, the words ‘comprise’,‘comprising’, ‘includes’, ‘including’ and the like are to be construedin an inclusive sense as opposed to an exclusive or exhaustive sense;that is to say, in the sense of “including, but not limited to”.

1. A method for the electrochemical reduction of dinitrogen to ammonia,the method comprising the steps of: (1) contacting a cathodic workingelectrode comprising a nanostructured catalyst and a counter electrodewith an electrolyte comprising one or more liquid salts, wherein theliquid salt is formed by a combination of (i) a cation selected from thegroup consisting of PR₁₋₄, NR₁₋₄, and C₄H₈NR₂ wherein each R group isindependently linear, branched, or cyclic and comprises from 1 to 18carbon atoms, optionally partially or completely halogenated, optionallyincluding a heteroatom, optionally including a functional group chosenfrom ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates,amines, azos, or nitriles, and wherein two R groups connect to form amonocyclic or heterocyclic ring; and (ii) an anion selected from thegroup consisting of (RO)_(x)PF_(6-x) (RO)_(x)BF_(4-x), R′SO₂NSO₂R′,R′SO₂C(SO₂R′)(SO₂R′), FSO₂NSO₂F, C₂O₄BF₂, C₂O₄PF₄, RC₂O₄BF₂, RC₂O₄PF₄,CF₃SO₃, R′SO₃, R′CO₂, CF₃COO, R′_(x)PF_(6-x), and R′_(x)BF_(4-x) whereineach R′ group is independently linear, branched, or cyclic and comprisesfrom 1 to 18 carbon atoms, optionally partially or completelyfluorinated and optionally including a functional group, chosen fromethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines,azos, or nitriles, and wherein two R′ groups connect to form amonocyclic or heterocyclic ring; (2) introducing dinitrogen and a sourceof hydrogen to the electrolyte, wherein the dinitrogen is reduced toammonia at the cathodic working electrode.
 2. The method according toclaim 1, wherein the liquid salt is formed by a combination of: (i) acation selected from the group consisting of C₄mpr, P_(6,6,6,14), andP(C₂R_(fn))₄ where R_(f) is a perfluoroalkyl moiety; and (ii) an anionselected from the group consisting of eFAP, NfO, PFO, FSI, NTf₂,B(otfe)₄, and CF₃COO.
 3. The method according to claim 1, wherein theliquid salt is selected from the group consisting of[P_(6,6,6,14)][eFAP], [C₄mpyr][eFAP], [P_(6,6,6,14)][F₉C₄SO₃],[P_(6,6,6,14)][PFO], [C₄mpyr][perfluorobutanesulfonate], and[C₄mpyr][PFO]. 4.-8. (canceled)
 9. The method according to claim 1,including the further step of raising the temperature of the electrolyteto between −35° C. and 200° C.
 10. The method according to claim 1,including the further step of subjecting the electrolyte to a pressurebetween 0.7 bar nitrogen to 100 bar nitrogen.
 11. The method accordingto claim 10, wherein the pressure is pulsed.
 12. The method according toclaim 1, wherein a current passing between the cathodic workingelectrode and the counter electrode is intermittent.
 13. The methodaccording to claim 1, further including the step of applying energy ofultrasonic frequency to the electrolyte.
 14. The method according toclaim 1, including the further step of humidifying dinitrogen gas, andpassing the humidified dinitrogen gas in a stream over the cathodicworking electrode.
 15. The method according to claim 1, wherein theelectrolyte further comprises a membrane chosen from a polymerelectrolyte, gelled ionic liquid electrolyte, or porous separator. 16.(canceled)
 17. A cell for electrochemical reduction of dinitrogen toammonia, the cell comprising: a cathodic working electrode comprising ananostructured catalyst for reduction of dinitrogen, a counterelectrode, and an electrolyte comprising one or more liquid salts incontact with the working electrode wherein the liquid salt is formed bya combination of: (i) a cation selected from the group consisting ofPR₁₋₄, NR₁₋₄, and C₄H₈NR₂ wherein each R group is independently linear,branched, or cyclic and comprises from 1 to 18 carbon atoms, optionallypartially or completely halogenated, optionally including a heteroatom,optionally including a functional group chosen from ethers, alcohols,carbonyls, thiols, sulphoxides, sulphonates, amines, azos, or nitriles,and wherein two R groups connect to form a monocyclic or heterocyclicring; and (ii) an anion selected from the group consisting of(RO)_(x)PF_(6-x), (RO)_(x)BF_(4-x), R′SO₂NSO₂R′, R′SO₂C(SO₂R′)(SO₂R′),FSO₂NSO₂F, C₂O₄BF₂, C₂O₄PR)RC₂O₄BF₂, RC₂O₄PF₄, CF₃SO₃, R′SO₃, R′CO₂,CF₃COO, R′_(x)PF_(6-x) (FAP), and R′_(x)BF_(4-x) wherein each R′ groupis independently linear, branched, or cyclic and comprises from 1 to 18carbon atoms, optionally partially or completely fluorinated andoptionally including a functional group, chosen from ethers, alcohols,carbonyls, thiols, sulphoxides, sulphonates, amines, azos, or nitrilesand wherein two R′ groups connect to form a monocyclic or heterocyclicring.
 18. The cell according to claim 17, wherein the liquid salt isformed by a combination of: (i) a cation selected from the groupconsisting of C₄mpyr, P_(6,6,6,14), and P(C₂R_(fn))₄ where R is aperfluoroalkyl moiety; and (ii) an anion selected from the groupconsisting of eFAP, NfO, PFO, FSI, NTf₂, B(otfe)₄, and CF₃COO.
 19. Thecell according to claim 17, wherein the liquid salt is selected from thegroup consisting of [P_(6,6,6,14)][eFAP], [C₄mpyr][eFAP],[P_(6,6,6,14)][F₉C₄SO₃], [P_(6,6,6,14)][PFO],[C₄mpyr][perfluorobutanesulfonate], and [C₄mpyr][PFO]. 20.-24.(canceled)
 25. The cell assembly formed by stacking in series two ormore cells according to claim
 17. 26. The cell according to claim 17,further including heating means to maintain the temperature of theelectrolyte between −35° C. and 200° C., preferably 0° C.
 27. The cellaccording to claim 17, further including pressurising means to subjectthe electrolyte to a pressure between 0.7 bar nitrogen to 100 barnitrogen.
 28. The cell according to claim 17, further adapted to pass anintermittent current between the cathodic working electrode and thecounter electrode.
 29. The cell according to claim 17, which furtherincludes a humidifier for humidifying the dinitrogen gas, and the cellfurther adapted to pass the humidified dinitrogen gas in a stream overthe cathodic working electrode.
 30. The cell according to claim 17,wherein the electrolyte comprises a membrane chosen from a polymerelectrolyte, gelled ionic liquid electrolyte, or porous separator. 31.(canceled)
 32. The cell according to claim 17, further including anultrasonic source for applying energy of ultrasonic frequency to theelectrolyte.
 33. The cell according to claim 17, wherein the reductionof dinitrogen to ammonia occurs principally in a region adjacent a threephase boundary on the working surface of the cathodic working electrode.34. The cell according to claim 17, wherein the nanostructuredelectrocatalyst is adjacent an outer surface of the cathodic workingelectrode and creates a gas/electrolyte/metal three phase boundaryregion where electrolysis principally takes place. 35.-40. (canceled)41. The cell according to claim 17, wherein the electrolyte furthercomprises a molecular liquid present in the liquid salt medium at alevel between 90 vol % and 0.1 vol %.
 42. The cell according to claim17, wherein the electrolyte further comprises a molecular liquid chosenfrom dimethylsulfoxide, tetraglyme and other oligio- and poly-ethers,glutaronitrile and other high boiling point nitriles, andtrifluorotoluene present in the liquid salt medium at a level between 90vol % and 0.1 vol %. 43.-45. (canceled)
 46. An electrolyte membranecomprising a thin layer of material in combination with one or moreliquid salts for use in the cell of claim 1, wherein the liquid salt isformed by a combination of: (i) a cation selected from the groupconsisting of PR₁₋₄, NR₁₋₄, and C₄H₈NR₂ wherein each R group isindependently linear, branched, or cyclic and comprises from 1 to 18carbon atoms, optionally partially or completely halogenated, optionallyincluding a heteroatom, optionally including a functional group chosenfrom ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates,amines, azos, or nitriles, and wherein two R groups connect to form amonocyclic or heterocyclic ring; and (ii) an anion selected from thegroup consisting of (RO)_(x)PF_(6-x), (RO)_(x)BF_(4-x), R′SO₂NSO₂R′,R′SO₂C(SO₂R′)(SO₂R′), FSO₂NSO₂F, C₂O₄BF₂, C₂O₄PF₄, RC₂O₄BF₂, RC₂O₄PF₄,CF₃SO₃, R′SO₃, R′CO₂, CF₃COO, R′_(x)PF_(6-x), and R′_(x)BF_(4-x) whereineach R′ group is independently linear, branched, or cyclic and comprisesfrom 1 to 18 carbon atoms, optionally partially or completelyfluorinated and optionally including a functional group, chosen fromethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines,azos, or nitriles and wherein two R′ groups connect to form a monocyclicor heterocyclic ring.
 47. The electrolyte membrane according to claim46, wherein the thin layer of material is chosen from the groupcomprising a polymer, a gel, or a porous separator material.
 48. Theelectrolyte membrane according to claim 46, wherein the liquid salt isin combination with one of a polymer, a gel, or a porous separator, or amaterial comprising a polymer, a gel, or a porous separator. 49.-50.(canceled)
 51. The method for the electrochemical reduction ofdinitrogen to ammonia according to claim 1 carried out using: anelectrochemical cell for electrochemical reduction of dinitrogen toammonia, the electrochemical cell comprising: a cathodic workingelectrode comprising a nanostructured catalyst for reduction ofdinitrogen, a counter electrode, and an electrolyte comprising one ormore liquid salts in contact with the working electrode wherein theliquid salt is formed by a combination of: (i) a cation selected fromthe group consisting of PR₁₋₄, NR₁₋₄, and C₄H₈NR₂ wherein each R groupis independently linear, branched, or cyclic and comprises from 1 to 18carbon atoms, optionally partially or completely halogenated, optionallyincluding a heteroatom, optionally including a functional group chosenfrom ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates,amines, azos, or nitriles, and wherein two R groups connect to form amonocyclic or heterocyclic ring; and (ii) an anion selected from thegroup consisting of (RO)_(x)PF_(6-x), (RO)_(x)BF_(4-x), R′SO₂NSO₂R′,R′SO₂C(SO₂R′)(SO₂R′), FSO₂NSO₂F, C₂O₄BF₂, C₂O₄PF₄, RC₂O₄BF₂, RC₂O₄PF₄,CF₃SO₃, R′SO₃, R′CO₂, CF₃COO, (FAP), and R′_(x)BF_(4-x) wherein each R′group is independently linear, branched, or cyclic and comprises from 1to 18 carbon atoms, optionally partially or completely fluorinated andoptionally including a functional group, chosen from ethers, alcohols,carbonyls, thiols, sulphoxides, sulphonates, amines, azos, or nitrilesand wherein two R′ groups connect to form a monocyclic or heterocyclicring; an electrolyte comprising one or more salts having dinitrogensolubility of at least 100 mg/L at the operating temperature andpressure of the electrochemical cell and having water solubility of lessthan 5 wt % at the operating temperature and pressure of theelectrochemical cell; and a nanostructured catalyst for reduction ofdinitrogen to ammonia comprising nanoparticles and having a highelectrochemical working surface area as indicated by a double layercapacity, measured in an adjacent electrolyte layer of greater than 0.1mF/cm² and preferably greater than 1 mF/cm².
 52. The method according toclaim 1, wherein the electrolyte further comprises a molecular liquidpresent in the liquid salt medium at a level between 90 vol % and 0.1vol %.
 53. The method according to claim 1, wherein the electrolytefurther comprises a molecular liquid chosen from dimethylsulfoxide,tetraglyme and other oligio- and poly-ethers, glutaronitrile and otherhigh boiling point nitriles, and trifluorotoluene present in the liquidsalt medium at a level between 90 vol % and 0.1 vol %.